This invention relates generally to the field of apparatus and methods for controlling the output of a light source having an optical output signal of tunable wavelength. More specifically, this invention relates to apparatus and methods for stabilizing and tuning the wavelength of the optical output signal from a coherent light source, such as a laser, used in a fiber optic system.
Wavelength stability in the optical output signal of the light source is necessary in many sensor systems and telecommunication systems using optical fibers, whether the light source is a superluminescent diode (SLD) or a laser (either a solid state laser or a semiconductor diode laser), especially when the scale factor of the system depends upon the source wavelength. For example, high precision, navigation grade fiber optic rotation sensors usually require a wavelength stability of about one part per million (ppm). A high degree of stabilization is also necessary in the lasers used in fiber optic telecommunication systems.
The wavelength of the light emitted from many types of lasers varies as a function of the operating temperature, and of the current applied to the energy source for excitation. Therefore, wavelength stabilization means must be employed to compensate for the effects of temperature and current variations, so that the suitable degree of stabilization is obtained.
A number of approaches to such light source stabilization have been developed, and are exemplified in several U.S. patents. For example, U.S. Pat. No. 4,842,358--Hall discloses optical signal source stabilization using an interferometer to form optical beams. The interferometer comprises a pair of birefringent crystals that are placed in the optical path of a light beam from an optical signal source. The crystals, having polarization-dependent refractive indices, produce a first beam having an intensity I.sub.o (1+cos.phi.) and a second beam having an intensity I.sub.o (1-cos.phi.). At the desired source frequency, the two intensities are equal. The difference between the two intensities is used to generate an error signal that is applied to servo the drive current of the source so as to produce an optical output signal from the source that minimizes the error signal. Alternatively, a fiber optic Mach-Zehnder interferometer may be used instead of the crystal interferometer.
U.S. Pat. No. 5,167,444--Hall discloses optical signal source stabilization, wherein the optical output signal from the source is stabilized by adjusting its frequency to maintain a selected optical transmission through a Fabry-Perot interferometer (or, more properly, an etalon). The interferometer or etalon has a "split level" gap; that is, the gap is split into two discrete portions of different widths. A photodetector associated with each portion generates an electric signal indicative of the beam intensity or power transmitted through that portion of the gap. At a desired wavelength of the optical signal incident on the interferometer, the transmission of light through the gap will be approximately equal in both gap portions, resulting in substantially equal amplitudes of the electrical signals from the two photodetectors. The two photodetector signals are fed into a differential amplifier, producing an error signal that is used, in a closed-loop servo, to control the drive current of the optical source so as to minimize the amplitude of the error signal. This minimum will occur at the desired wavelength.
While the above-described systems are capable of yielding satisfactory results, they exhibit some limitations. For example, manufacturing costs tend to be high: The system of U.S. Pat. No. 4,842,358, supra, requires relatively expensive components, such as birefringent crystals made to precisely controlled tolerances; while the fabrication of the "split level" gap of the system of U.S. Pat. No. 5,167,444, supra, is relatively difficult and and is not easily adapted to multiple light source applications. Moreover, the above-described systems lack a suitable tuning capability, which may be desired in some applications.
There has been an unsatisfied need, therefore, for a system of light source wavelength stabilization that is more easily and economically fabricated than the prior art systems, and that also has a convenient and accurate tuning capability.